1. Field of the Invention
The present invention is directed to a method for measuring the length of an electric cable.
2. Description of the Related Art
The length of a cable can play a critical role in the transmission of the signals carried thereby. WO 2010/126467 discloses a method for indicating the input cable length for serial digital interface signals. The method comprises measuring a differential that varies between first (+1 V) and second values (−1 V) approximately linearly in proportion to a characteristic of a signal at an end of the cable, and translating the measured differential voltage into a cable length measurement.
WO 2010/092256 describes a device for measuring the length of a moving cable provided with marks, which are fixed at predefined intervals along the cable. The device comprises electronic detection means, which are suitable for automatically detecting, on the moving cable, every local change in the transverse geometry linked to the presence of a mark.
Logging operations in oil and gas wells in general require accurate determination of the location of the logging tool in the borehole of the well. A way to accomplish the accurate location determination of the logging tool during logging operations involves monitoring the downhole length of the cable or wireline supporting the logging tool payed into or reeled out of the well bore by the cable draw works. Monitoring of the length can be performed by using a calibrated count wheel which is matched precisely to the diameter of the wireline or cable loaded onto the cable drum/reel of the draw works to generate a distance of travel signal for the length of cable payed out or reeled in. U.S. Pat. No. 6,745,487 describes a downhole cable length measuring apparatus comprising three major components: a rotational distance measuring assembly; a rotation-to-length, calibration assembly, and a processor unit.
Cable length sensors of a cable that is wound on a cable drum have been used. Generally, the cable is fastened on one of its end sides and is pre-tensioned by a return element. Since the diameter of the cable drum is known, the cable length drawn off can be determined by means of the sensor through an evaluation unit, which is usually external. US 2008/0141548 relates to a cable length sensor in which the return device is disposed in a housing and coupling means for coupling the return device to the direction of turning of the cable drum is provided. Fixing means, which can be actuated from outside of the housing and which serves for fixing the current position of the return device, in particular the at least slightly pre-tensioned starting position of the return device, is associated with the housing.
In some applications, customers can be charged for cable and installation by the total length purchased or installed. In such cases, it can be useful to measure length of cable after installation without the requirement to measure it beforehand. US 2006/181283 discloses a cable diagnostic mechanism that utilizes time domain reflectometry (TDR) to detect and identify cable faults, perform estimations of cable length, identify cable topology, identify load and irregular impedance on metallic paired cable, such as twisted pair and coaxial cables.
Since the measurement made using TDR is performed on electric conductors, when the conductors are stranded around the cable axis—a very common configuration—the length of the conductor is higher than the length of the cable, and this difference is not known with satisfactory accuracy because the lay up pitch is not exactly controlled.
WO 08/073,033 describes a system for monitoring the bending and strain of a power cable connected to a moving offshore platform by measuring the strain in optical fibres attached to or incorporated into the power cable. A bend in the power cable will give rise to a strain in the optical fibre and this strain will change the optical properties of the fibre. The change in optical properties can be measured by means of optical time domain reflectometer (OTDR) or optical frequency domain reflectometer (OFDR).
GB 2368921 discloses a wellbore cable having spaced apart ends including a first end and a second end, a hollow metal tube extending from the first end to the second end thereof, and at least one fibre optic loosely disposed within the hollow metal tube and extending therein from the first end to the second end thereof. The at least one fibre optic has least one fibre Bragg grating.
EP 0203249 discloses a medium-voltage (from 6 to 60 kV) power cable that includes at least one temperature and/or tension sensor optical fibre.
Typically, electric cables are delivered to customers coiled about bobbins together with the information of the nominal length of the wound cable. In some applications, such as for use in a down well environment, it can be important to know the “real” length of the payed off cable so as to define the route of the cable along the deployment. Apparently negligible lengths, such as 1 meter or less, especially when missing, can give rise to inconvenience.